This invention concerns polymeric and non-polymeric binders for particles and the use of such binders in binding particles to fibers. The fibers and bound particles may be easily densified by external application of pressure. The binders are reactivatable, and may be applied to particles, which thereafter may be bound to fibers on a wet-laid fiber sheet manufacturing line. In particular embodiments, the invention concerns binding superabsorbent particles to cellulosic fibers which may then be used, for example, to make absorbent fibers that are densified and incorporated into absorbent products. In other embodiments, the invention concerns the reactivation and use of binder coated particles, preferably at an article manufacturing plant, at a location remote from a pulp-sheet manufacturing line to bind superabsorbent and/or other particles to cellulosic fibers which may then be used, for example, as absorbent fibers incorporated into absorbent products.
Superabsorbent polymers have been developed in recent years that are capable of absorbing many times their own weight of liquid. These polymers, which are also known as water insoluble hydrogels, have been used to increase the absorbency of sanitary products such as diapers and sanitary napkins. Superabsorbent polymers are often provided in the form of particulate powders, granules, or fibers that are distributed throughout absorbent cellulosic products to increase the absorbency of the product. Superabsorbent particles are described, for example, in U.S. Pat. No. 4,160,059; U.S. Pat. No. 4,676,784; U.S. Pat. No. 4,673,402; U.S. Pat. No. 5,002,814; and U.S. Pat. No. 5,057,166. Products such as diapers that incorporate absorbent hydrogels are shown in U.S. Pat. No. 3,669,103 and U.S. Pat. No. 3,670,731.
One problem with the use of superabsorbents is that the superabsorbent material can be physically dislodged from the cellulosic fibers of an absorbent product. Separation of the superabsorbent from its substrate reduces the absorbency of the product and diminishes the effectiveness of the superabsorbent material. This problem was addressed in European Patent Application 442 185 A1, which discloses use of a polyaluminum chloride binder to bind an absorbent polymer to a fibrous substrate. The polyaluminum binder, however, suffers from the drawback of being an inorganic product that is not readily biodegradable. Moreover, that European patent does not offer any guidance for selecting binders other than polyaluminum chloride that would be useful in binding absorbent particles.
A method of immobilizing superabsorbents is disclosed in U.S. Pat. No. 4,410,571 in which a water swellable absorbent polymer is converted to a non-particulate immobilized confluent layer. Polymer particles are converted to a coated film by plasticizing them in a polyhydroxy organic compound such as glycerol, ethylene glycol, or propylene glycol. The superabsorbent assumes a non-particulate immobilized form that can be foamed onto a substrate. The individual particulate identity of the superabsorbent polymer is lost in this process. The confluent nature of the superabsorbent material can also result in gel blocking, in which absorption is diminished as the water swollen polymers block liquid passage through the film layer.
U.S. Pat. No. 4,412,036 and U.S. Pat. No. 4,467,012 disclose absorbent laminates in which a hydrolyzed starch polyacrylonitrile graft copolymer and glycerol mixture is laminated between two tissue layers. The tissue layers are laminated to each other by applying external heat and pressure. The reaction conditions form covalent bonds between the tissue layers that firmly adhere the tissue layers to one another.
Numerous other patents have described methods of applying binders to fibrous webs. Examples include U.S. Pat. No. 2,757,150; U.S. Pat. No. 4,584,357; and U.S. Pat. No. 4,600,462. Such binders are not described as being useful in binding particulates, such as superabsorbent particles, to fibers. Yet other patents disclose crosslinking agents such as polycarboxylic acids that form covalent intrafiber bonds with individualized cellulose fibers, as in European Patent Application 440 472 A1; European Patent Application 427 317 A2; European Patent Application 427 316 A2; and European Patent Application 429 112 A2. The covalent intrafiber bonds are formed at elevated temperatures and increase the bulk of cellulose fibers treated with the crosslinker by forming intrafiber ester crosslinks. Crosslinking must occur under acidic conditions to prevent reversion of the ester bonds. The covalent bonds within the fibers produce a pulp sheet that is more difficult to compress to conventional pulp sheet densities than in an untreated sheet. Covalent crosslink bonds may also form between the fibers and particles, and occupy functional groups that would otherwise be available for absorption, hence absorption efficiency is decreased.
A particular disadvantage of forming covalent ester intrafiber crosslinks is that the resulting fiber product resists densification. Energy requirements for making densified absorbent products are increased because very high compression pressures must be used to densify the absorbent product. It would be advantageous to provide a method of enhancing densification of crosslinked fibers by reducing energy requirements for densification.
Many different types of particles other than superabsorbents may be added to fibers for different end uses. Antimicrobials, zeolites and fire retardants are but a few examples of particles that are added to fibers. It would be advantageous to provide a method of attaching particles that could be accommodated to the many different particle needs of end users. Moreover, it would be advantageous to reduce particulate waste in the attachment process, and simplify shipment of fiber products that require particulate addition. It would be further advantageous to bind particulates to fibers without requiring the shipment of bulk fibers with adhered particulates because shipping and excessive handling of these fibers subject them to mechanical impact which can dislodge some particles from the fibers. It would also be advantageous under some circumstances to incorporate binder coated particles onto fibers during the initial pulp sheet manufacturing process so that the fibers with particles are ready for use at a remote product manufacturing location. However, the particles are then subject to dislodgement during the subsequent manufacturing processes.
It has previously been important that particles added to cellulose products be insoluble in liquids such as water or liquid binders. It has been thought that liquid insolubility (particularly water insolubility) was an essential characteristic for particles bound to cellulose fibers because soluble particles would be dissolved by a water containing binder. Although the particle could eventually resolidify as the binder evaporated, dissolution of the particle in the binder would cause the particle to diffuse to areas of the product where it was not needed or desired. Water soluble particles have therefore not been used for particles that were to be bound to fibers using a binder.
The foregoing and other problems have been overcome by providing fibers with hydrogen bonding functional sites, and binders that have a volatility less than water. The binder has a functional group that forms a hydrogen bond with the fibers, and a functional group that is also capable of forming a hydrogen bond or a coordinate covalent bond with particles that have a hydrogen bonding or coordinate covalent bonding functionality. The binder is applied to the particles to at least partially coat the particles. The binder containing particles, when combined with the fibers, are bonded to the fibers by a bond that has been found to be resistant to mechanical disruption. A significant advantage of these binders is that the binder and particle together on the fiber have been found to reduce the pressure required to densify the fibers. This is particularly true for superabsorbent particles, and preferably comprises using superabsorbent particles and a binder in an active state. The binders can also be present on particles in an inactive state for more than a week, a month, or a even a year, then later activated or reactivated to bind particles to the fibers. Liquid binders (which includes neat liquids or aqueous solutions of solid binders) can be placed on the particles, dried, and later reactivated by moistening the particles. Alternatively, a dry solid binder may be blended with the particles and later activated by addition of a liquid. An inactive binder can also be activated by applying kinetic energy to the binder containing particles in the presence of the fibers. Typically, an inactive state is one where the binder and particles reach an equilibrium moisture content with the atmosphere (hereinafter referred to as xe2x80x9cair dryxe2x80x9d). Kinetic energy can be applied to the binder and fibers, for example and without limitation, by applying mechanical agitation, pressure from an external source, or using ultrasonics. In yet other embodiments, the binder may be activated or reactivated by heating the binder containing particles after applying the binder to the particles.
The capacity for activation or reactivation allows the binder to be applied to the particles, which are then shipped to distribution points with the binder in an inactive form. The binder is then activated at the distribution point (for example, a customer""s facility) where binder containing particles are added to the fibers and bound thereto. As used herein, binder xe2x80x9cactivationxe2x80x9d includes both activation of previously inactive binders (such as solid binders in the absence of liquid) or reactivation of previously active binders (such as a liquid binder that has been air dried). More typically, the particles are exposed to the binder, e.g. by spraying binder onto a stream of particles, as the particles are being deposited on a web of fibers or are otherwise being combined with the fibers. The binder binds the particles to the fibers. By applying binder primarily to the particles instead of directing the binder to the fibers, lesser quantities of binder are required to bind the particles to the fibers.
Another advantage of the present invention is that the binder containing particles can be applied to a fiber product in a pattern that corresponds to a desired distribution of particles in fibrous material. The binder may then be reactivated to bind the particles in place. A reactivation fluid, such as a reactivation liquid, for example, can be applied to binder containing particles deposited in the areas of a diaper that will be initially moistened by urine during use. Examples, without limitation, of suitable reactivation liquids include water, glycerin, lower alkyl alcohols, polyols, such as glycols, glycerin and monoglycerates, acetone, and combinations thereof, such as water and glycerin. When the reactivating fluid is a liquid, for example water, the liquid may be sprayed or otherwise applied and may be provided in the form of a gas. When water is the reactivating liquid, then it may be provided as steam or moisture-laden gas, such as humid air. Other liquid reactivation fluids may be applied in the same manner. Binder containing particles, such as superabsorbent particles can be added to areas of the diaper to which an activation fluid is or will be applied and will be adhered almost exclusively in those areas where initial urine absorption is required. Targeted activation of binder containing particles allows the particles to be efficiently and economically attached to the fibers, with reduced particle wastage. Moreover, targeted binder activation and particle adherence increases the absorptive efficiency of the product by diminishing excessive wicking of liquid within the plane of an absorptive product.
The particles of the present invention may be bound to the fibers with a polymeric or non-polymeric binder. The binder comprises binder molecules wherein the binder molecules have at least one functional group capable of forming a hydrogen bond or coordinate covalent bond with the particles, and at least one functional group capable of forming a hydrogen bond with fibrous material. The polymeric binder may be selected from the group consisting of polyglycols (especially polyethylene glycol or poly(propyleneglycol), a polycarboxylic acid, a polycarboxylate, a poly(lactone) polyol, such as diols, a polyamide, a polyamine, a polysulfonic acid, a polysulfonate and combinations thereof. Specific examples of some of these binders, without limitation, are as follows: polyglycols include polypropylene glycol (PPG) and polyethylene glycol (PEG); poly(lactone) polyols include poly(caprolactone) diol;, polycarboxylic acids include polyacrylic acid (PAA); polyamides include polyacrylamide or polypeptides; polyamines include polyethylenimine and polyvinylpyridine; polysulfonic acids or polysulfonates include poly(sodium-4-styrenesulfonate) or poly(2-acrylamido-methyl-1-propanesulfonic acid); and copolymers thereof (for example a polypropylene glycol/polyethylene glycol copolymer). The polymeric binder typically has repeating units. The repeating unit may be the backbone of a compound, such as with a polypeptide, wherein the repeating polyamides occur in the peptide chain. The repeating unit may also refer to units other than backbones, for instance a repeating acrylic acid unit. In such a case, the repeating units may be the same or different. The binder has a functional group capable of forming a hydrogen bond or a coordinate covalent bond with particles, and a functional group capable of forming a hydrogen bond with the fibers. At this time, the most preferred polymeric binder is polyethylene glycol although another especially preferred polymeric binder is an amide binder such as a polypeptide binder with polyglycine being a specifically preferred example.
The non-polymeric binder has a volatility less than water, a functional group that is capable of forming a hydrogen bond or coordinate covalent bond with the particles, and a functional group that is capable of forming a hydrogen bond with the cellulose or other fibers. The non-polymeric binder is an organic binder, and preferably includes a functional group selected from the group consisting of a carboxyl (for example, carboxylic acids), a carboxylate, a carbonyl (for example, aldehydes), a sulfonic acid, a sulfonate, a phosphoric acid, a phosphate, an amide, an amine, a hydroxyl (such as an alcohol) and combinations thereof (for example, an amino acid or an hydroxy acid), wherein there are at least two functionalities on the molecule selected from this group, and the two functionalities are the same or different. Examples of such binders include polyols, polyamines (a non-polymeric organic binder with more than one amine group), polyamides (a non-polymeric organic binder with more than one amide group), polycarboxylic acids (a non-polymeric organic binder with more than one carboxylic acid functionality), polyaldehydes (a non-polymeric organic binder with more than one aldehyde), amino alcohols, hydroxy acids and other binders. These binders have functional groups that are capable of forming the specified bonds with the particles and fibers.
More preferably, the organic non-polymeric binder is selected from the group consisting of glycerin, a glyceride monoester, including monoglycerides, a glycerin diester, including diglycerides, glyoxal, ascorbic acid, urea, glycine, pentaerythritol, a monosaccharide or a disaccharide, citric acid, tartaric acid, taurine (2-aminoethanesulfonic acid), p-aminosalicylic acid, dipropylene glycol, urea derivatives such as DMDHEU, and combinations thereof. Suitable saccharides include glucose, sucrose, lactose, ribose, fructose, mannose, arabinose, and erythrose. The preferred binders are non-polymeric molecules with a plurality of hydrogen bonding functionalities that permit the binder to form hydrogen bonds to both the fibers and particles. Particularly preferred binders include those that can form five or six membered rings, most preferably six membered rings, with a functional group on the particle surface. At present, glycerin, a glycerin monoester, a glycerin diester, and blends of these with urea are the preferred binders. At this time, a specifically preferred non-polymeric binder is glycerin.
The fibrous material may be cellulosic or synthetic fibers that are capable of forming hydrogen bonds with the binder, while the particles are selected to be of the type that are capable of forming hydrogen bonds or coordinate covalent bonds with the binder. It has unexpectedly been found that this binder system secures particles to fibers exceptionally well. A superior fibrous-product is therefore produced that has improved absorbent properties as compared to unbound or covalently bound particles. Formation of the noncovalent bond allows production of a fiber product that is easily manufactured and a web that is easily densified, and that is readily biodegradable and disposable.
In one preferred embodiment, an absorbent product comprises a fibrous cellulosic mat that contains superabsorbent hydrogel particles in particulate form. The superabsorbent particles are capable of forming hydrogen bonds or coordinate covalent bonds with the binder, depending upon the binder, while the binder in turn is capable of forming hydrogen bonds with the hydroxyl groups of the cellulose fibers. These noncovalent, relatively flexible bonds between the binder and particles maintain the particles in contact with the fibers, and resist dislodgement of the particles by mechanical forces applied to the mat during manufacture, storage or use. The amount of binder present typically depends on a number of factors, including the nature of the binder and particles, and whether the particles are immediately added to the fibers or after a period of time. Hence, one skilled in the art will realize that the amount of binder suitable and particularly useful for a particular application will vary. However, the binder may suitably be present in an amount of from about 0.01 to 50 percent of the total weight of the particles, preferably from 0.03 to 20 percent, more preferably 0.03 to 5 percent and most preferably 0.03 to 1 percent. This lower percentage range produces very strong bonds that would require a much higher quantity of binder if the binder were applied to the fibers instead of the particles. The particles bound by the binder of the present invention (via hydrogen/coordinate covalent bonds) may suitably be present in an amount of 0.05 to 80 percent of the total weight of the fibrous material and the particles, preferably 1 to 80 percent or 5 to 80 percent, or more than 5 percent by weight. A particularly suitable range of particles is 5 to 70 percent by weight of the fibrous material and particles. A preferred weight ratio of particle to binder is 90:1 to 500:1. An example of a suitable particle is a superabsorbent polymer such as a starch graft polyacrylate hydrogel fine or larger size particle such as a granule, which is capable of forming hydrogen bonds with the binder. The binder also is capable of forming hydrogen bonds with the hydroxyl groups of the cellulose, thereby securely attaching the superabsorbent particles to the fibers.
The present invention also includes a method of binding particles to fibers wherein the particles are insoluble in the binder (and soluble in water) and therefore retain their solid particulate form following binding. The particles, whether water soluble or not, preferably have functional groups that can form hydrogen bonds or coordinate covalent bonds with the binder, and the binder in turn is capable of forming hydrogen bonds to the fibers. Other particles without the desired functionality may also be included in the fiber product, but such particles will not be bound as strongly in the same manner.
In especially preferred embodiments, the fibers are cellulosic and the particles are superabsorbent particles that are bound to the binder by hydrogen bonds. The fibers also may be continuous or discontinuous synthetic or natural fibers having a hydrogen bonding functional group that hydrogen bonds with the binder. The binder is suitably applied to the particles in an amount of at least 0.03 percent, and preferably no more than 80 percent, more preferably no more than 20 percent and most preferably 0.1 to 3 percent, by total weight of the particles. The particles may be bound to the fibers at less than 150xc2x0 C. or without any external application of heat at ambient temperature (e.g., about 25xc2x0 C.). Particles may also be bound in the absence-of any external application of pressure, or in the absence of external heat and pressure.
In some embodiments the binder is associated with the particles as a solid (for example, a dry powder or a dried liquid), and the fibers contain at least 7 percent water by weight when the binding step is performed. This level of moisture in the fibers provides sufficient mobility of reactants to allow the particles and fibers to bind well to each other. When a liquid binder is used (for example, glycerin or a solution of glycine powder), the fibers suitably contain at least about 0.5 percent water by weight. A solid binder is suitably used with fibers having less than 0.5 percent water by weight if the binder is heated above its melting point to liquefy it. A solid binder may be thermoplastic or meltable, such that it can be heated above its melting point and then cooled to fuse fibers to each other. The thermoplastic properties of the binder can also provide additional mechanical adherence between the particles and fibers. In some embodiments, a meltable binder such as urea may be employed which can adhere particles both physically and with hydrogen bonding.
In other embodiments, the particles are soluble in water but have reduced solubility in the binder such that the particles can be bound in solid particulate form to the fibers. Addition of the binder does not dissolve the particle and cause it to diffuse away from its desired site of attachment to the fibers.
The invention also is directed to fibrous products produced by any of the methods described herein and to absorbent products or articles comprised of such fibrous products.